DescriptionA large and flat ranid containing protruding eyes, numerous dorsal warts, and a minute inner metatarsal tubercle. It has complete webbing between the toes and fingers.A large dorsoventrally flattened ranid frog with a very warty skin whose eyes and nostrils are dorsally positioned (when viewed from above the eyes are contained within the outline of the head). Due to numerous glands, the skin is very slippery. SVL of adult males: 52–104 mm. SVL of the largest females measured in Comoé National Park was 65 mm (but see below). Weight of males is 24–84 g (x= 42.7), of females: 20–132 g (x= 53.5). Broad mouth. The border of the lower jaw has three tooth like structures. Oval vertical pupil. The distinct tympanum is bordered by a bulging supratympanal fold. It reaches 0.7–0.9 of the eye diameter. Males have paired lateral vocal sacs and considerably enlarged thenar tubercles. The skin of the throat surrounding the vocal sac slits is heavily folded. The warts on the flanks are usually arranged in longitudinal rows. 6–8 of these rows are very distinct on the backs of young specimens, too. The hind limbs are comparatively short but very muscular. Thighs and shanks reach 0.4–0.5 of the SVL, each. The shanks are usually longer than the thighs. The foot, incl. the longest toe, reaches 0.6–0.8 of the SVL. Many small tubercles on the posterior part of the thighs. The very tiny inner metatarsal tubercle just reaches 0.2–0.4 of the shortest toe length. Webbing is complete. The hands with trace of webbing.A female measured by Loveridge (1955a) weighed 235 g at a SVL of 127 mm. Perret (1966) gives 68–110 (SVL) mm for males and 110–135 mm for females. The record is probably held by a female measuring 160 mm (see Schiøtz 1964a).Voucher specimens: SMNS 8956 1–5 + tadpoles; SMF: 78630–32.Coloration: The basic color of body and limbs is a yellowish green, olive or drab brown. Large dark green to blackish spots which occasionally form rows, are present on the back. Spots of the same color are present on the upper lip and on the extremities. A light green to yellow transverse line behind the eyes is always present in young frogs and usually in adult individuals, too. The outer parts of the thighs are marbled. The venter is white, sometimes marked with few black spots. The lower lips and the throat are mostly white, bearing very rarely some black spots. The vocal sacs are dark gray, light blue (Burton 1972) or cream (Loveridge 1955a). Böhme (1978), cites frogs with typical color pattern, and specimens whose basic color was brick red. In alcohol the venter turns drab gray. The patches are at most a little darker than the surrounding areas, remaining rather distinct on the extremities. The light transverse line behind the eyes remains visible.The advertisement call is a slightly rising and rather low gnarling sound which lasts about 0.2 sec. It consists of several pulses, each lasting 0.01 sec, and separated by intervals of the same length. As all recordings have been jammed by the voices of other species, its frequency can only be estimated: it probably ranges from 0.2 to 2.1 kHz. Its dominant frequency seems to range from 0.8–1.2 kHz. If danger is imminent, young frogs will utter high-pitched cheeping sounds. This observation is confirmed by Amiet (1974b). The call published by Van den Elzen & Kreulen (1979) is almost identical: it also lasts 0.2 sec and consists of 12 pulses, its dominant frequency being 1–1.5 kHz. The intervals between calls last 1.1–1.4 sec.Spawn: The eggs are attached singly or in small groups at the bottom or on stones in shallow water zones. The sticky jelly is soon covered by algae and detritus so that the eggs are hardly visible. The eggs diameter is approx. 5 mm. The egg diameter at the neurula stage is 3.2 mm.Zug (1987) measured ovarian eggs. They measured 1.6–1.8 mm and already had black and white poles. Loveridge (1925) gives 2 mm for ovarian eggs. Parker (1936c) and Sanderson (1936) described the clutch as an enormous mass of floating eggs. However, these authors have almost certainly confused Hoplobatrachus eggs with those of another species (probably Phrynomantis microps).The water temperature of the shallow ponds occasionally rose to 40 °C, particularly in rock-pools. In these habitats the tadpoles, which still have external gills at this stage, usually hatch within half a day. They measure 6.5 mm TL and 3 mm BL. In the aquarium, larvae reared at 30 °C and feeding on customary fish food grew very slowly. The oral disc began to develop 54 hours after hatching. Neither horny teeth nor nostrils were present at this stage. The external gills were still visible. At this stage the tadpoles measured 9.4mm (TL) and 4.2 mm (BL), respectively. At the age of 70 hours, they measured 12.7 mm (TL; BL: 4.7 mm). The external gills had been reduced, and the nostrils and oral disc were visible. Under natural conditions, their development may be completed within less than two weeks, but it often takes much longer, particularly in larger ponds.In natural ponds, the tadpoles already show their characteristic oval shape after one day. Nostrils and eyes are placed dorsally on the snout and front. The oral disc is surrounded by bulging lips and has the following keratodont formulae: 2 / 1+1 // 1+1 / 2 or 2 / 2+2 // 2+2 / 2. The most caudad teeth are arranged in a double row. The horny teeth have only one tip. The horny beaks have a characteristic form: the upper one is serrated, and its center shows a conspicuous hook; the lower one is voluminous and serrated, and it shows two pointed extensions. The space between these two extensions exactly match the hook of the upper jaw. Short blunt papillae border upper and lower lips. The body shape of well-fed larvae may be almost heart-shaped. The basic color is light beige with few dark spots. The dorsal tail fin inserts behind the level of the vent. Both fin sections are of almost equal width and transparent to feebly mottled.One of the largest tadpole showing no traces of hind limbs measured 18 mm (BL; TL: 52 mm). Much larger tadpoles have been observed. The hind legs are developed at a BL of 18–25 mm and a TL of 50–71 mm. The forelimbs emerge on tadpoles of the same size range. Freshly metamorphosed young measure 24–50 mm (SVL). The following exemplary data are meant to illustrate their development (size in mm / weight in g): 46 / 0.93; 48 / 1.05; 53 / 1.46; 58 / 1.60; 60 / 1.76; 63 / 2.12.The description of the tadpole published by Lamotte & Zuber-Vogeli (1954a) exactly matches the above given description. These authors give an additional keratodont formula: 4 + 4 // 2+2 / 2. They report on young frogs metamorphosing at a SVL of 29–30 mm.

Life History, Abundance, Activity, and Special BehaviorsIn the dry season, all H. occipitalis apparently concentrate on the river banks, seeking refuge under overhanging roots, in thick brush, in caves, under stones and in rock crevices. Spieler & Linsenmair (1998) studied this species in Comoé National Park over several years in the early rainy season. In the transition from dry to rainy season, high daily maximum temperatures are typical. The frogs were only active at night. From sunrise until sunset they stayed in their hiding places. The home ranges in the dry river bed ranged from 23–3744 m² (x= 142 m²). Frogs with home ranges along rivers had a higher body weight and smaller home ranges than on rocky stretches. They migrated when precipitation was higher than 15 mm (> 30 m per animal). Long distance movements (> 600 m) occurred after filling of the savanna ponds. Single specimens migrated a linear distance of up to 1.4 km in a single night. Spieler & Linsenmair (1998) calculated an activity area, including river and savanna ponds, of up to 2 km² for both sexes. With the aid of radio tracking they documented site fidelity of up to two weeks. Typical retreat sites were under patches of grass (beside savanna ponds) on land because of evaporative cooling. This is not necessary in the rainy season when H. occipitalis seems to stay mainly in water. Spieler & Linsenmair (1998) further documented intraspecific aggressive interactions. This might be the reason why smaller frogs had to settle in marginal habitats such as the rocky river bank. As soon as shallow rock-pools and puddles near the banks are filled with water after the first rainfalls, choruses congregate at night. Calling males either float in deeper water or sit in shallow water. Van der Elzen & Kreulen (1979) describe calling males, which continually change their positions, while floating at the water surface.The spawning behavior has been investigated by Spieler (1997b) and Spieler & Linsenmair (1997). Amplectant pairs, move from one rock-pool to the next, deposing just a few eggs in each of them. This strategy is meant to minimize the risks caused by predators and desiccation. Besides, the frogs are capable of predicting the waterholding capacity of a pond, regardless of its current water volume (Spieler & Linsenmair (1997). As the former more frequently depends on the ground-water level or on fine crevices in the substrate, but rarely on the water volume, it can hardly be estimated correctly by any human observer. However, ponds keeping their water for a very long time are obviously preferred. The frogs behavior possibly results either from choosing traditional spawning sites or from continuously monitoring the waterholding capacity of the respective waters. Furthermore, those ponds already harboring tadpoles of the same species are avoided. The larvae are carnivorous, preying preferably on tadpoles of other species. As the rainy season progresses and the level of the river rises, the rock-pools vanish. From then on, the frogs spawn in large savanna ponds. Later on, smaller ponds are also accepted. The females either cannot verify whether conspecific tadpoles are present in larger pools, or the risk of cannibalism is reduced in those waters. I have observed females spawning repeatedly in savanna ponds, even if conspecific tadpoles were present. The distribution of the tadpoles on different ponds is possibly not only meant to avoid the risks caused by cannibalism, but also to minimize the competition for food resources, which might be less important in larger ponds. There H. occipitalis arenever abundant, but evenly distributed across the bank zones.Kühn et al. (1987) showed that H. occipitalis females showed an annual variation in the reproductive capacity, even under nearly constant environmental conditions (photoperiod, temperature, precipitation, humidity). Reproductive females were however found throughout the year with a minimal activity from October to January (dry season lasts from December to February). A long lasting breeding season has been described by Amiet (1974b).In the aquarium, H. occipitalis tadpoles of varying size and collected in ponds far apart never killed each other during my experiments. The animals were kept in order to observe the methods employed in capturing their prey, after starving the tadpoles for two days. Conspecific tadpoles in adjacent aquariums were attacked "across" the panes as soon as they moved. It therefore seems likely that capture behavior is triggered by optical cues, and conspecific tadpoles are identified by smell. These very effective and fast-growing predators are mainly found in the shallow bank zones of ponds. Heavily grown pond areas are as readily occupied as those lacking any vegetation. The tadpoles often surface to breath atmospheric oxygen. They are capable of killing tadpoles up to 1/3 larger and whose weight surpasses their own. Larger tadpoles are more likely to survive an attack than small ones (Rödel 1998b). Their chances of survival also depend on their locomotive behavior and preferred microhabitats (Rödel 1998b). As soon as a Hoplobatrachus tadpole has located a potential prey item, it will orient towards the latter: the tail slightly vibrates before the predator rushes at the victim. Small tadpoles are immediately swallowed in one piece whereas larger ones are seized by the powerful beaks. The prey is then gradually chewed, and the liquids are sucked in. During these violent attacks, large pieces of tissue are expulsed through the spiracle of the predator. If the blood vessels of the victim are damaged, H. occipitalis tadpoles immediately turn red.Both young and adult frogs are almost exclusively aquatic. The former are particularly observed during the day, floating at the surface with their limbs stretched out. Adult frogs often bask on the banks. Disturbed animals rush over the surface of the water for a short time when fleeing, another distinctive feature of this species. Young frog are mainly found in smaller ponds where they are most probably less subject to the risk of being preyed on by older frogs of their own species. In Senegal, Böhme (1978) found adult and subadult frogs in different habitats: the former inhabited residuary puddles, and the latter were located, partially mummified, in the crevices developing at the bottom of dried-up pools.Young frogs often fall victim to large aquatic bugs (Belostomatidae) and dragonfly larvae, but adult animals are frequently captured, too. Spieler (1997c) has shown by means of radio tracking that potential predators include the nile monitor (Varanus niloticus) and a soft-shelled turtle (Cyclanorbis senegalensis).On May 5th, 1995, we found a deformed young frog showing three complete right forelegs and a stump at the same place. Most probably, the tadpole had been injured while the forelimbs were still developing, and several arms had been regenerated simultaneously.In Senegal, the diet of this frog mainly consists of beetles, spiders and orthopterans (Lescure 1971). These agile predators select their prey according to its speed, movements and size. The considerable proportion of beetles in the diet most probably indicates that the frogs usually live near waters (Inger & Marx 1961, Rödel 1995). Loveridge (1942) found a frog in the stomach contents of one specimen. Noble (1924) even cites conspecifics as prey. Kühn et al. (1987) lists other ranids, insects and fish as prey items. According to these authors H. occipitalis stays in water during daytime. It leaves water for hunting at night, but remains in water in the full-moon period. In North Africa H. occipitalis is active day and night (Salvador 1996). They feed on coleopterans and orthopterans, toads (Bufo xeros) and their tadpoles. During winter they are inactive.

CommentsThis account was taken from Rödel, M.-O. (2000), Herpetofauna of West Africa vol. I. Amphibians of the West African Savanna, with kind permission from Edition Chimaira publishers, Frankfurt am Main.For references in the text, see here

Written by M.O. Roedel (roedel AT biozentrum.uri-wuerzburg.de), Post-Doc at the University of Wurzburg, Department of Animal Ecology and Tropical Biology, Wurzburg, GermanyFirst submitted 2001-05-04Edited by Arie van der Meijden and VTV (2002-01-14)